The present invention relates to ink supply for inkjet printers and, in particular, it concerns the subdivision of the volume of an ink supply reservoir into two or more inter-connected sub-volumes, to alleviate pressure fluctuations caused by acceleration of the reservoir.
It is known to employ an arrangement of one or more inkjet print heads in a scanning motion to achieve full print coverage across the length and width of a substrate. For certain applications, it is important to minimize the size and weight of the moving components and consequently, ink is supplied to the print head from a stationary ink reservoir. However, for other applications this is impractical, and the print head and reservoir move together on a print head assembly and hence execute the same motion. In order to achieve rapid printing, large velocities over short distances, and therefore large accelerations, are required of the print head and reservoir.
FIG. 1 shows a cross-sectional side view of a typical reservoir 600 and indicates a container which is composed of walls, including side walls 601, a base 602 containing supply outlets 603, and a lid 604 containing an atmosphere communication opening 605, which together define a contained volume 608. If the reservoir is accelerated in the primary direction of motion 609, i.e. to the right, this results in a gradient of the ink level 607 where the magnitude of the gradient is proportional to the magnitude of the acceleration. Clearly, a similar, but opposite, situation would arise if the acceleration was in the opposite direction, i.e. to the left.
The liquid ink gradient induces a pressure gradient across the base of the reservoir, where the pressure above each ink supply outlet 603, is proportional to the height of liquid ink above it. Furthermore, when reservoir 600 stops accelerating, gravity causes ink level 607 to tend towards the horizontal, thereby causing fluctuations in ink level 607. Fluctuations of ink level 607 are highly undesirable for two main reasons. Firstly, effective operation of the print head (not shown) relies on a small and constant pressure differential between the print head and reservoir 600. Small pressure fluctuations, on the order of millimeters of ink, between the print head and reservoir 600 can dramatically degrade the print quality. Secondly, if ink level 607 fluctuations become excessive, and the resulting pressure fluctuations exceed some threshold, this can result in total cessation of printing.
Another problem associated with acceleration is the inertial force acting on the liquid ink within the tubes, making up supply lines, connecting a print head and reservoir which are not aligned on the same plane normal to the direction of the acceleration. This scenario is depicted schematically in FIG. 2, which shows a top view of a print head assembly 509. Print head assembly 509 is made up of an ink reservoir 500, a print head 502 and a rigid plate 506. Ink reservoir 500 and print head 502 are mounted rigidly on rigid plate 506. Print head assembly 509 accelerates in a primary direction of motion 507. A supply line 501, from a supply outlet 503 of reservoir 500, supplies ink to print head 502 via a print head supply inlet 505. The distance between supply outlet 503 and supply inlet 505, parallel to primary direction of motion 507, is indicated 504. When print head assembly 509 accelerates, an inertial force acts on the ink within supply line 501 which is proportional to length 504. The inertial force in the supply line 501 causes a pressure fluctuation in print head 502 that is proportional to length 504. Much like the example presented in FIG. 1, pressure fluctuations of this nature can be severe enough to dramatically reduce print quality or even bring about cessation of printing.
There is therefore a need for an ink reservoir for supplying ink to an inkjet print head which will overcome the aforementioned problem of pressure fluctuations. Specifically, there is a need to eliminate or significantly reduce reservoir ink level fluctuations which arise during print head assembly accelerations. Associated with this problem, there is a need to eliminate pressure fluctuations resulting from inertial forces acing on ink in supply lines during print head assembly accelerations.